1. Field of the Invention
The present invention relates to a track detector circuit for detecting a track when at least either optical recording or regenerating of information is performed on an information recording/regenerating medium having a plurality of tracks each of which includes at least a first area formed by combining areas of different reflectances along tracks and a second area formed by combining areas of different reflectances perpendicularly to the tracks.
2. Description of the Related Art
An optical card has the same size and shape as a credit cart, which is portable but not rewritable similarly to an optical disk. The optical card is characterized by the large recording capacity (1 to 2.5 megabytes) and low costs per sheet of card. The optical card is therefore recognized as a promising information recording/regenerating medium for the next generation, for which a wide range of applications are pondered over.
An optical card shown in FIG. 20 has been proposed as an example of a conventional optical information recording/regenerating medium in Japanese Patent Laid-Open No.63-145669. In the optical card 1, information is recorded as pits and regenerated optically as differences in reflectances. The information is recorded or regenerated in units of what is referred to as a track 153. Multiple linear band-like tracks 153 are lined up in parallel with one another, thus forming a sheet of card. ID areas 152a, 152b, and 152c are allocated to both ends and center of the tracks 153. Track number patterns, and specific identification patterns allocated to the respective tracks for identifying the track numbers are pre-recorded in the ID areas, thus simplifying search for a target track.
Additionally-recorded information and pre-recorded information are saved in data areas 151a and 151b shown in FIG. 20.
FIG. 21 shows the detail of a track 153 in an area A enclosed with a dotted line in FIG. 20. Guide patterns 155 that are black and white patterns formed at regular intervals in the longitudinal direction of the card (along tracks) are pre-recorded in a guide line area 154 serving as a first area located in the center of the track 153. Data of 8 by 8 bits totaling to 16 bits are recorded across the track (seek direction) with the guide line area 154 between them.
As for the ID area 152a, alphanumeric characters 156a to 156h denote patterns for representing a track number and alphanumeric characters 157a to 157h denote specific identification patterns allocated to every track. The track number patterns 156a to 156h and identification patterns 157a to 157h constitute a second area.
The track number patterns 156 and identification patterns 157 in the ID area 152a are detected during track search (coarse seek). The data consisting of 16 bits in the data area 151 are read concurrently during regenerating. Thus, fast reading is achieved.
FIG. 22 shows an overall construction of an optical head 3. A semiconductor laser (LD) 10 is driven only for recording, while a light emitting diode (LED) 9 is driven for recording and regenerating.
Light originating from the LED 9 is irradiated onto an optical card 1 after passing through a collimator lens 12, a dichroic mirror 13a, a half prism 14, and an objective lens 15, and then illuminates broadly the entire width of a track. Light reflected from the optical card 1 is received by a photodetector 8' after passing through the objective lens 15, half prism 14, and an image formation lens 16.
Modulated light from the semiconductor laser 10 passes through a collimator lens 21, the dichroic mirror 13a, the half prism 14, and the objective lens 15, and then becomes a microscopic recording beam which will be projected on the optical card 1 within the broadly-illuminated region. The light projected on the optical card 1 physically changes the recording surface of the optical card 1 so as to record information. Light reflected from the optical card 1 is received by the photodetector 8' after passing through the objective lens 15, half prism 14, and image formation lens 16.
A recording beam is aligned across tracks by rotating the dichroic mirror 13a using a rotating means which is not shown. The objective lens 15 is driven along the optical axis and across tracks with driving current induced in a focus coil 26 and a tracking coil 27 respectively. Specifically, a focus servo and a tracking servo are actuated so that a recording or regenerating light spot comes into focus on the optical card 1 and follows a track.
FIG. 23 shows a light receiving surface 8a of the photodetector 8'. An image of the track 153 shown in FIG. 21 is formed on the light receiving surface 8a', and then a detected signal is output. The photodetector 8' includes 16 light receiving elements for data reading 8-A1 to 8-A16 associated with data recording positions of 16 bits which are arranged across tracks, and five pairs of light receiving elements for clock production 8-B1' to 8-B10' which are arranged along tracks and separated from one another so as to receive the light of the images of the guide patterns 155. The photodetector 8' further includes four pairs of light receiving elements for servo signal detection 8-C1 to 8-C4 and 8-D1 to 8-D4 which are arranged across tracks being separated from one another and opposed mutually.
FIG. 24 shows a distribution of quantities of the light originating from the LED 3 and forming an image on the photodetector 8'. When a relative distance of the optical card i from the objective lens 15 varies, the distribution of quantities of light originating from the LED 9 and forming an image on the photodetector 8' changes as shown in FIG. 24. A focus error (FE) signal is produced on the basis of a difference between a sum of the outputs of the outer light receiving elements for servo signal detection 8-C1, 8-C2, 8-C3, and 8-C4 and a sum of the outputs of the inner light receiving elements 8-D1, 8-D2, 8-D3, and 8-D4. A tracking error (TE) signal is produced on the basis of a difference between a sum of the outputs of the light receiving elements for servo signal detection 8-D1 and 8-D3, and a sum of the outputs of the light receiving elements 8D2 and 8-D4.
A clock signal is produced on the basis of a difference between a sum of the outputs of the odd-numbered light receiving elements 8-B1' 8-B3', 8-B5', 8-B7', and 8-B9', and a sum of the outputs of the even-numbered light receiving elements 8-B2', 8-B4', 8-B6', 8-B8', and 8-B10'. An optical card recording/regenerating apparatus having the foregoing optical head 3 reads 16-bit data concurrently from the outputs of the light receiving elements for data reading 8-A1 to 8-A16 during regenerating. During recording, a recording beam originating from the semiconductor laser 10 is projected to record data.
FIG. 25 shows the wave of a TE signal generated when the optical head 3 is moved across tracks with a tracking servo system broken.
When a target track is searched for, the recording/regenerating point of a light spot is aligned with the center of an image of a guide patterns 155 in the target track. For this alignment, for example, Japanese Patent Laid-Open No. 2-203818 has proposed a tracking servo leading method to be mentioned below.
In the proposal, as shown in FIG. 26, an on-track (or off-track) signal is produced depending on whether or not a 16-bit regenerative signal based on the target track number patterns 156 and identification patterns 157 is consistent. After a tracking servo loop is closed (TON is high), a TE signal is selected in an on-track region (corresponding to a substantially linear portion of the TE signal), and a specified bipolarity voltage Va or Vb is selected in an off-track region (corresponding to a portion of the TE signal slightly deviated from an extreme value thereof). Thus, tracking servo leading is achieved.
However, when the recording/regenerating point of the optical head is aligned with the center of a specific track having the guide patterns 155 arranged at specified intervals along the track and multiple information lines located with the guide patterns 155 between them, the aforesaid 16-bit regenerative signal has been used to produce an on-track signal in the past.
Any of eight bits of the track number patterns 156 or the identification patterns 157 may be detected incorrectly due to dust, a flaw, a defect, of the like. Or when a regenerative signal is locked in synchronization with the zero-line crossing of the TE signal, a noise may be placed on the regenerative signal. Under these circumstances, an on-track signal is not output at a target position, a specified voltage Va or Vb is therefore not selected in an off-track region, and eventually a TE signal alone is used to achieve leading.
Consequently, the swing (inertia force) of an objective lens becomes stronger than the braking force of a servo system. Eventually, tracking servo leading fails. If a regenerative signal has not fully been binary-coded, an on-track signal is not output. This also results in a failure in tracking servo leading.